For some pathogens, attack is the best form of defense—they enter immune cells of the human body. However, if they are detected in their hidden niche, the infected cell kills itself to re-expose the pathogens. In the EMBO ...

Chemists at the University of Basel have succeeded in using computer simulations to elucidate transient structures in proteins. In the journal Angewandte Chemie, the researchers set out how computer simulations of details ...

In a phenomenon known as mechanochromism, certain solid and liquid crystalline materials change their photoluminescence properties upon mechanical stimulation, such as grinding, ball-milling and crushing. Although such compounds ...

An enhanced design for a promising computer memory technology has been developed by A*STAR researchers. Victor Zhuo and colleagues developed resistive random-access memory (RRAM) that, during fabrication, does not require ...

In an advance that helps pave the way for next-generation electronics and computing technologies—and possibly paper-thin gadgets —scientists with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley ...

Physicists from New Zealand's University of Otago have used steerable 'optical tweezers' to split minute clouds of ultracold atoms and slowly smash them together to directly observe a key theoretical principle of quantum ...

During equilibration ordinary many-body systems lose all information about the initial state. Every morning we experience an example for this behaviour. Milk poured into a cup of coffee mixes perfectly and after some time ...

Russian scientists have identified the components of the oldest bitumen sample to be found in an ancient vase and have made an accurate estimate of its age. In an article in the Journal of Mass Spectrometry, the research ...

On December 31, 2016, a "leap second" will be added to the world's clocks at 23 hours, 59 minutes 59 seconds Coordinated Universal Time (UTC). This corresponds to 6:59:59 pm Eastern Standard Time, when the extra second will ...

Atom

The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.

The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom.

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus, with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.